(1) Field of the Invention
The present invention relates to an intraocular lens, more particularly to a soft silicone intraocular lens having a high refractive index and an ultraviolet absorbability closer to that of the human lens.
(2) Prior Art
As the material for intraocular lenses, there has been mainly used polymethyl methacrylate (hereinafter referred to as PMMA) as described in, for example, Japanese Patent Application Kokai (Laid-Open) No. 59444/1978. With intraocular lenses made of a hard material represented by PMMA, however, various problems cannot be overcome such as (1) these lenses give a strong mechanical irritation to the corneal endothelium, uveal tissue and other ocular tissues at the time of or after lens implantation and (2) a size of incision at least equal to the diameter of the optic (ordinarily 6-7 mm) must be formed at the time of lens implantation and thereby delay in physical rehabilitation and postoperative astigmatism are caused.
Hence, studies have been made on an intraocular lens made of a soft material, which gives a weak mechanical irritation to the corneal endothelium, uveal tissue and other ocular tissues and which can be inserted into the eye through an incision smaller than the diameter of the optic. As a result, there was developed an intraocular lens made of a silicone elastomer and then an intraocular lens made of hydrogel, for example, poly-2-hydroxyethyl methacrylate (hereinafter referred to as PHEMA). The silicone intraocular lens described in, for example, U.S. Pat. No. 4573998 has advantages such as (1) being soft and flexible, (2) having good heat resistance and accordingly being autoclavable, which cannot be applied to PMMA intraocular lenses, and (3) having been widely used as a medical material and providing excellent safety. The PHEMA intraocular lens in a hydrogel state after hydration has advantages such as (1) being soft and flexible, (2) being autoclavable similarly to the silicone elastomer, and (3) showing relatively low adsorption for protein, etc.
The human lens has an ability of absorbing an U.V. light which is harmful to the retina. Therefore, in recent years, there have been made attempts to allow an intraocular lens to have an U.V. absorbability in order to protect the retina of a patient to whom an intraocular lens has been implanted. As a result, there was developed a hard PMMA intraocular lens having an U.V. absorbability, as described in, for example, Japanese Patent Application Kokai (Laid-Open) No. 38411/1985.
However, each of the above mentioned soft materials for intraocular lens which were developed in order to solve the problems of the hard materials such as PMMA, has drawbacks. In the case of the silicone elastomer, it is said that since the dimethylpolysiloxane as a main component of the silicone elastomer has a specific gravity fairly smaller than that of PMMA, the intraocular lens made of the silicone elastomer, tends to cause decentration, tilting and dislocation in the eye. Further, the silicone elastomer has a refractive index as low as 1.405 posing a limitation in the optical design of intraocular lens. That is, the radius of curvature necessary for obtaining a desired refractive power is small; as a result, the resulting intraocular lens has a large thickness, is difficult to insert into the eye, and tends to form wrinkles when folded; this makes it difficult to sufficiently utilize the advantage of a soft intraocular lens that the lens can be inserted through an incision smaller than the diameter of the optic.
Furthermore, the silicone elastomer has substantially no U.V. absorbability; therefore, an U.V. absorber must be added in a large amount in order to endow the silicone elastomer with a desired U.V. absorbability; however, it is difficult to uniformly disperse a generally used U.V. absorber in the silicone elastomer because such an U.V. absorber is difficultly soluble in dimethylpolysiloxane.
In the case of PHEMA, it has a low strength and, in some cases, breaks during the surgical operation for inserting an intraocular lens made of PHEMA into the eye. Further, since PHEMA is a hydrogel, the intraocular lens made thereof intakes aqueous humor, which may cause the staining and discoloration due to adhesion and deposition; thus, the intraocular lens made of PHEMA has also a problem in long-term stability in the eye.
In intraocular lenses having an U.V. absorbability, if the lenses contain a large amount of an U.V. absorber, the U.V. absorber which is harmful to the ocular tissues, may dissolve in aqueous humor in an increased amount. In the conventional intraocular lenses comprising PMMA and an U.V. absorber, the amount of the U.V. absorber used has been as high as 0.15% by weight and accordingly the amount of the U.V. absorber dissolving in aqueous humor has been innegligible. When an U.V. absorber has a low solubility in an intraocular lens material to which the absober is to be added and when an intraocular lens comprising said material and said absorber has been implanted into the eye, it is generally thought that the absorber dissolves in aqueous humor in an increased amount and that it may give an adverse effect to the ocular tissues.